1. Field of the Invention
The present invention relates to a noise shielding apparatus in which a noise generated in a magnetron of a microwave oven can be effectively shielded. More particularly, the present invention relates to a noise shielding apparatus in which the structure of the shielding apparatus is simple, but the performance is very high, so that the productivity can be improved, and that the overall manufacturing cost can be saved.
2. Prior Arts
Generally, various apparatus such as home microwave ovens, commercial thawing apparatuses, industrial driers and the like using microwaves are provided with a magnetron for generating microwaves, and a capacitor for shielding noises.
In an electric field room of a microwave oven, there is provided a magnetron for generating microwaves. Such microwaves are generated when a high voltage produced by primary and secondary induction coils of a high voltage transformer which is attached on a base plate of the electric field room, is stably supplied to the magnetron, the high voltages being generated through the inductive interaction between the induction coils. Such microwaves are irradiated into a cooking chamber through an irradiating tube.
When the microwaves are irradiated into the cooking chamber after passing through the irradiating tube, the food placed within the cooking chamber is heated so as to be cooked.
The power supply line of the magnetron is mainly consisted of a filament, a cathode and an anode. When the high voltage is supplied to the magnetron to generate microwaves, unnecessarily radiated microwaves, i.e., noises are generated, besides microwaves having basic frequencies which are suitable for heating the food. Then, the noises flow back through the filament and the cathode so as to cause wave obstructions in the nearby apparatuses.
Particularly, coming recently, television broadcasts resorting to satellites are widely utilized. The unnecessary microwaves of the magnetron interacts with the broadcasting frequencies and therefore there is a possibility that receiving disorders may occur on a television receiver.
In order to reduce such adverse influences given to the nearby apparatuses due to the magnetron noise, a choke coil and a capacitor connected thereto are provided on the cathode which supplies power to the filament. The choke coil which has a reactance, and the capacitor which is connected to the choke coil absorb the unnecessary microwaves, thereby blocking the leakage of the unnecessary microwaves.
The choke coil is sealed within a shielding case which is provided under the magnetron, while the capacitor is installed on the outside of the shielding case. One end of the choke coil is connected to the power supply line of the filament, while the other end is connected to a lead line of the capacitor.
The widely used capacitor is a through-type, and such a through-type capacitor is described in U.S. Pat. No. 4,811,161 (issued to Sasaki et al). In the magnetron using the through-type capacitor, the choke coil is connected in series between the cathode of the magnetron and a through conductor of the through-type capacitor which is inserted in a side wall of the shielding case.
FIG. 1 is an exploded perspective view of a noise shielding apparatus including a conventional through-type capacitor 30 and FIG. 2 is a front sectional view of through-type capacitor 30 of FIG. 1.
As shown in the drawings, the conventional through-type capacitor 30 includes an elliptic ceramic dielectric 32. Ceramic dielectric 32 is provided with a pair of vertical through holes 34 which are formed in parallel with each other. On the upper surface of ceramic dielectric 32, there are provided a pair of electrodes 36 which are separated from each other, while a common electrode 38 is provided on the lower surface of ceramic dielectric 32. Separated electrodes 36 and common electrode 38 are provided with through holes corresponding to through hole 34 of ceramic dielectric 32. Capacitor 30 further includes a ground fitment 40 made of a metal in which an elliptic opening 42 is formed at a center portion thereof, on which there is formed an upstand 44 along the circumference of opening 42 with a suitable height. Ceramic dielectric 32 is fixed via common electrode 38 on upstand 44 of ground fitment 40 by a proper means such as soldering or the like.
Further, capacitor 30 includes a pair of through conductors 46 each covered with an insulation tube 48, insulation tube 48 being formed of a suitable material such as silicon. Insulation tube 48 is inserted into through hole 34, and opening 42 and through conductor 46 each are fittedly secured in an electrode connectors 50 each of which is fixed on separated electrodes 36 by a proper means such as soldering or the like. Fixing of through conductor 46 to electrode connector 50 may be carried out by soldering or the like.
Ground fitment 40 is formed by pressing a metal plate in such a manner that upstand 44 should surround opening 42 in a projected contour, and that the other side of ground fitment 40 is provided with a recess 52 so as to form the inner surface of upstand 44. At the four corner portions of ground fitment 40, there are formed four piercing holes 41, so that ground fitment 40 may be attached to a shielding case (which is also called a "filter box") 90.
Capacitor 30 further includes an insulation case 54 which surrounds ceramic dielectric 32 and an insulation cylinder 56 which surrounds through conductor 46. The lower portion of insulation case 54 is secured to upstand 44 of ground fitment 40, while the upper portion of insulation cylinder 56 is secured by recess 52 of ground fitment 40. Insulation case 54 and insulation cylinder 56 are filled with an insulation resin material such as an epoxy resin or the like so as to cover an outside and inside of ceramic dielectric 32 with the resin or embed it therein to thereby ensure moisture proofness and insulation properties of ceramic electric. Insulation case 54 and insulation cylinder 56 are made of thermoplastic resin such as polybutylene terepthalate (PBT).
Each through conductor 46 is integrally provided with a fastening tab 62 on one end thereof which is to be received into insulation case 54 for applying a high voltage. One end of fastening tab 62 projects from one end of insulation case 54, so that the tab can be easily connected to an external terminal.
When ground fitment 40 is fixedly secured on shielding case 90, shielding case 90 is provided with a large hole 91 corresponding to the capacitor and four bearing holes 92 corresponding to four piercing holes 41 of ground fitment 40. Then bearing holes 92 and piercing holes 41 are matched to assemble them using bolts.
Through-type capacitor 30 which connects the choke coil of the interior of shielding case 90 with an external terminal inhibits the conducting noise from conducting through the lead, and also shields a radiating noise. However, as shown in the drawings, the conventional noise shielding apparatus of a magnetron includes many components assembled together, and therefore, not only the structure is complicated so as to increase the material cost, but also the assembling process is very fastidious so as to lower the productivity. Further, after the assembling, a considerable amount of radiating waves is leaked through insertion hole 91 of shielding case 90, holes 41 of ground fitment 40 and burring holes 92 of shielding case 90, with the result that the shielding of the noise cannot be maximized.
Therefore, magnetron manufacturers are expending efforts to simplify the structure of the capacitor, or to design a new structure in replace of that of the conventional capacitor.